What is the O.P.C.W., and Can It Referee on Chemical Weapons in Syria?

The group describes itself as “an independent, autonomous, international organization with a working relationship with the United Nations.”

In 2013, Syria signed the convention and agreed to eliminate its chemical weapons stockpiles. On a joint mission with the United Nations, the organization arranged for the transport of all chemical weapons the Syrian government had declared, for disposal overseas.

Over 96 percent of state-declared stockpiles around the world have been destroyed under the watch of the organization. However, as seems to be the case in Syria, that doesn’t necessarily mean that countries no longer have chemical weapons.

Collecting samples while making sure they can be used for evidence takes time, and such missions can turn investigators into targets. When inspectors deployed to Syria in 2014, for example, their convoy came under fire.

Last year, while looking into further allegations of chemical weapons use in Syria, investigators did not visit the town of Khan Sheikhoun because of security fears. Instead, they relied on witness accounts and samples collected from the site. That opened their conclusions to criticism from Russia and Syria, which contended that Damascus had disposed of all its chemical weapons.

Can the O.P.C.W. point the finger?

Not anymore. Not officially, at least. The organization’s job is to establish whether chemical weapons were used, not who used them.

“It’s dealing with things it wasn’t really intended to deal with,” said Richard Guthrie, a chemical weapons expert and editor of CBW Events, a website that tries to document uses of chemical and biological weapons.

When the Chemical Weapons Convention was being negotiated, Mr. Guthrie said, “the concern was large-scale use of chemical weapons on the battlefield — that had happened in the Iran-Iraq war.” But that is different from identifying the relatively small-scale use of chemical weapons, like the alleged case in Syria, or the attack on the spy and his daughter in Britain.

Until the end of last year, the organization had a mandate to pass on its findings to a Joint Investigative Mechanism, established by the United Nations Security Council, which would try to identify the perpetrators of attacks. But last year Russia vetoed the extension of that mandate.

That leaves a disconnect: Even if the organization finds that chemical weapons were used in Douma, the question of who is to blame could remain unresolved.